In the tweet he says he is using https://elevenlabs.io for the voice, which is currently the best quality text-to-speech service and allows for voice cloning too.

And Elevenlabs is saying this about cloning other people's voices:

Can I clone anybody's voice?

Yes, as long as you have their consent. For Professional Voice Cloning, we have integrated robust security measures to make sure you can only clone your own voice. Unless you share it, your voice belongs and is available only to you.

But it looks like it's not enforced if the example in the tweet is possible.

But even if it would be enforced by the company, open-source models are getting close to them and they are harder to control. The public or commercial use of the output from these models can (and probably should) be limited by laws, but if the users want to use it privately (for example to narrate themselves) there is not much you can do to prevent it especially with the progress in this tech it will become more trivial to do it for anybody.